Public networks, and most notably the Internet, are emerging as a primary conduit for communications, entertainment, and business services. The Internet is a network formed by the cooperative interconnection of computing networks, including local and wide area networks. It interconnects computers from around the world with existing and even incompatible technologies by employing common protocols that smoothly integrate the individual and diverse components.
The Internet has recently been popularized by the overwhelming and rapid success of the World Wide Web (WWW or Web). The Web is a graphical user interface to the Internet that facilitates interaction between users and the Internet. The Web links together various topics in a complex, non-sequential web of associations which permit a user to browse from one topic to another, regardless of the presented order of topics. A "Web browser" is an application which executes on the user's computer to navigate the Web. The Web browser allows a user to retrieve and render hypermedia content from the WWW, including text, sound, images, video, and other data.
One problem facing the continued growth and acceptance of the Internet concerns dissemination of streaming continuous data, such as video and audio content. Data is delivered and rendered to users in essentially two formats. The first format, referred to as "block data," entails downloading the entire data set to local storage and then rendering the data from the locally stored copy. A second delivery format, known as "streaming data," entails sending bits of data continuously over the network for just-in-time rendering.
Computer network users have been conditioned through their experiences with television and CD-ROM multimedia applications to expect instantaneous streaming data on demand. For technical reasons, however, the Internet is often unable to deliver streaming data. This inability is most pronounced for video data. In the Internet context there is often long delays between the time video content is requested and the time when the video content actually begins playing. It is not uncommon to wait several minutes for a video file to begin playing. In essence, for factors discussed below, video data is traditionally delivered as "block data" over the Internet and thus requires that the entire file be downloaded prior to rendering.
The inability to provide streaming data is a result of too little bandwidth in the distribution network. "Bandwidth" is the amount of data that can be moved through a particular network segment at any one time. The Internet is a conglomerate of different technologies with different associated bandwidths. Distribution over the Internet is usually constrained by the segment with the lowest available bandwidth.
FIG. 1 shows a model of a public network system 20, such as the Internet. The network system 20 includes a content server 22 (e.g., a Web server) which stores and serves multimedia data over a distribution network 24. The network system 20 also has regional independent service providers (ISPs) or point of presence (POP) operators, as represented by ISP 26, which provide the connectivity to the primary distribution network 24. Many users, as represented by subcriber computers 28, 30, and 32, are connected to the ISP 26 to gain access to the Internet.
The ISP 26 is connected to the distribution network 24 with a network connection 34. In this example illustration, the network connection 34 is a "T1" connection. "T1" is a unit of bandwidth having a base throughput speed of approximately 1.5 Mbps (Megabits per second). Another common high bandwidth connection is a T3 connection, which has a base throughput speed of approximately 44.7 Mbps. For purposes of explaining the state of the technology and the practical problems with providing real-time streaming data over the Internet, it is sufficient to understand that there is also a limited bandwidth connection between the content server 22 and the distribution network 24.
The subscriber computers 28, 30, and 32 are connected to their host ISP 26 via a home entry lines, such as telephone or cable lines, and compatible modems. As examples of commercially available technology, subscriber computer 28 is connected to ISP 26 over a 14.4K connection 36 which consists of a standard telephone line and a V.32bis modem to enable a maximum data rate of 14.4 Kbps (Kilobits per second). Subscriber computer 30 is connected to the ISP 26 with a 28.8K connection 38 (telephone line and V.34 modem) which supports a data rate of 28.8 Kbps. Subscriber computer 32 is connected to the ISP 26 with an ISDN connection 40 which is a special type of telephone line that facilitates data flow in the range of 128-132 Kbps. Table 1 summarizes connection technologies that are available today.
TABLE 1 ______________________________________ Connection Technologies and Throughput Connection Type Base Speed (Kbps) ______________________________________ V.32bis modem 14.4 V.34 modem 28.8 56K Leased Line 56 ISDN BRI (1 channel) 56-64 ISDN BRI (2 channels) 128-132 Frame Relay 56-1,544 Fractional T1 256-1,280 ISDN PRI 1,544 Full T1 (24 channels) 1,544 ADSL 2,000-6,000 Cable Modem 27,000 T3 44,736 ______________________________________
With a T1 connection to the primary distribution network 24, the ISP 26 can facilitate a maximum data flow of approximately 1.5 Mbps. This bandwidth is available to serve all of the subscribers of the ISP. When subscriber computer 28 is connected and downloading data files, it requires a 14.4 Kbps slice of the 1.5 Mbps bandwidth. Subscriber computers 30 and 32 consume 28.8 Kbps and 128 Kbps slices, respectively, of the available bandwidth.
The ISP can accommodate simultaneous requests from a number of subscribers. As more subscribers utilize the ISP services, however, there is less available bandwidth to satisfy the subscribers requests. If too many requests are received, the ISP becomes overburdened and may not be able to adequately service the requests in a timely manner, causing frustration to the subscribers. If latency problems persist, the ISP can purchase more bandwidth by adding additional capacity (e.g., upgrading to a T3 connection or adding more T1 connections). Unfortunately, adding more bandwidth may not be economically wise for the ISP. The load placed on the ISP typically fluctuates throughout different times of the day. Adding expensive bandwidth to more readily service short duration high-demand times may not be profitable if the present capacity adequately services the subscriber traffic during most of the day.
The latency problems are perhaps the most pronounced when working with video. There are few things more frustrating to a user than trying to download video over the Internet. The problem is that video requires large bandwidth in comparison to text files, graphics, and pictures. Additionally, unlike still images or text files, video is presented as moving images which are played continuously without interruption. Video typically requires a 1.2 Mbps for real-time streaming data. This 1.2 Mbps throughput requirement consumes nearly all of a T1 bandwidth (1.5 Mbps). Accordingly, when multiple subscribers are coupled to the ISP and one subscriber requests a video file, there is generally not enough capacity to stream the video in real-time from the content server 22 over the Internet to the requesting subscriber. Instead, the video file is typically delivered in its entirety and only then played on the subscriber computer. Unfortunately, even downloading video files in the block data format is often inconvenient and usually requires an excessive amount of time.
Consider the following example. Suppose a subscriber wishes to access the CNN Web site on the Internet for an account of recent news. As part of the news materials, CNN provides a twenty second video clip of an airplane hijacking incident. At 1.2 Mbps, the 20 second video clip involves downloading a 24 Mbyte file over the Internet. If the user has a modest 14.4 Kbps connection, it would take approximately 28 minutes to download the entire file.
Now, assume that the subscriber/ISP connection is sufficiently large to handle real-time video streaming of the video file, meaning that the subscriber computer can render the video data as it is received from the ISP. Despite the bandwidth of the subscriber/ISP connection, real-time video streaming may still be unachievable if the T1 connection 34 between the ISP 26 and the distribution network 24 is unable, or unwilling due to policy reasons, to dedicate 1.2 Mbps of its bandwidth to the video file. Requests for the CNN video clip made during peak traffic times at the ISP most certainly could not be accommodated by the ISP/network connection. Since adding more bandwidth may be a poor investment for the ISP, the ISP may have no economic incentive to remedy the latency problem. The result is that some users might be inconvenienced by the lack of ability to receive streaming video despite their own connection to the ISP being capable of accommodating streaming video.
The latency problem is further aggravated if the connection between the content server 22 and the distribution network 24 is equally taxed. The lack of sufficient bandwidth at the content server/network link could also prevent real-time video streaming over the Internet, regardless of the bandwidths of the network/ISP link or the ISP/subscriber link. If all links lack sufficient bandwidth, the latency problem can be compounded.
One solution to this problem is to provide local cache storage at the ISP. As subscribers request files from the Internet, the ISP caches the files locally so that subsequent requests are handled in a more expeditious manner. This process is known as "on-demand caching." Local on-demand caching methods improve the ability to deliver video content over the Internet. When the first subscriber requests the CNN video clip of the airplane hijacking incident, the ISP requests the video clip from the CNN server, and facilitates delivery of the video clip to the requesting subscriber. The ISP also caches the video clip in its own memory. When any subsequent subscriber requests the same CNN video clip, the ISP serves the local version of the video clip from its own cache, rather than requesting the clip from the CNN server. If the subscriber computer has a high bandwidth connection with the ISP, the locally stored video clip can be served as continuous streaming video data for instantaneous rendering on the subscriber computer.
A drawback of the on-demand caching method is that the first requesting subscriber is faced with the same latency problems described above. All subsequent subscribers have the benefit of the cached version. However, if the initial delay is too long, there may not be any subscriber who is willing to assume the responsibility of ordering the video file and then waiting for it to download.
Accordingly, there remains a need to develop improved techniques for facilitating distribution of streaming video over public networks, such as the Internet.